Process for preparing filamentary microtapes of labyrinthian cross section



June 1969 J. R. FROST 3,448,187

PROCESS FDR PREPARING FILAMENTARY MICROTAPES OF LABYRINTHIAN CROSS SECTION Filed Aug. 26, 1966 Fig. 1

INVENTOR. John R. Pros? 42 wai /0,4,

rQTTOR/VE Y6 United States Patent PROCESS FOR PREPARING FILAMENTARY MICROTAPES OF LABYRINTHIAN CROSS SECTION John R. Frost, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Aug. 26, 1966, Ser. No. 575,281 Int. Cl. B29c 17/02 US. Cl. 264285 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for preparing filamentary microtapes of labyrinthian cross section from flat tapes and ribbons. More particularly, it relates to such a process which provides exceptional control of the width, denier, and cross-sectional configuration of said microtapes.

For many applications and uses, it is desirable to have filamentary articles of other than the traditional solid cylindrical configuration. For example, flat tapes and ribbons would seem to be a suitable form for the manufacture of many woven and unwoven fabrics and similar articles. Such tapes and ribbons provide unusually high covering power per unit of volume and thus result in fabrics of less weight and of greater flexibility than those fabrics prepared from the prior filamentary articles of cylindrical section. However, when tapes or ribbons are employed in the usual weaving operations, it is usually necessary to twist, to plait, or otherwise to change the character of the flat tape or ribbon prior to weaving. Those operations result in a filamentary article which is characterized by poor control of Width, denier, and crosssectional geometry and result in fabrics having a hand that is little better than those prepared from the conventional cylindrical filaments.

One filamentary article that is finding increasingly wide usage in many applications is a filamentary microtape that is essentially rectangular, oval, or elliptical in peripheral silhouette. In cross section, however, these useful microtapes consist of a continuous transverse section of a tape or ribbon having the edges curled or having a transverse section of the tape or ribbon folded two, three, or more times back upon itself resulting, in effect, in a series of layers separated by air. This curling or folding results in a microtape having a labyrinthian cross section. These microtapes of such labyrinthian section may be used directly in conventional textile operations and, when so used, are characterized in providing the high covering power per unit of volume inherent in such peripheral silhouette. In addition, these microtapes are relatively bulky due to the air spaces between the layers.

These filamentary microtapes of labyrinthian cross section, when prepared from the known organic thermoplastic resinous materials, are of great desirability for several reasons, in addition to those just mentioned. Tapes and ribbons from which the desired microtape may be formed are readily prepared. Thermoplastic microtapes permit the preparation of fabrics which may be thermally formed, heat sealed, embossed, calendered, and the like. However, the thermoplastic materials frequently require orientation if the optimum properties are to be achieved in the filamentary article. In many cases it is necessary or at least desirable to the very attainment of the tape or ribbon to orient the tape or ribbon prior to fabrication. However, the oriented form of a polymer is subject to certain difficulties, such as fibrillation in fiat tape form, when that flat tape is subjected to great stress. It would be desirable to have a process for preparing oriented filamentary microtapes of labyrinthian cross section from continuous, coherent tapes and ribbons.

A method for preparing such microtapes is disclosed in US. Patent 2,858,186, issued Oct. 28, 1958, on an application filed by J. R. Frost. In that patent there is disclosed a process wherein a latex of a polymer is continuously coagulated, the so-formed coagulum slit into fiat tapes, the slit tapes dried, fused, shaped, and oriented. However, reliance solely on an inherent tendency to curl or to fold, as in that patent, results in filamentary microtapes of wide width variation.

It has been suggested that labyrinthian microtapes having improved width control can be prepared by the method of 1) the passage of a flat, flexible tape of an organic thermoplastic polymer into heated restricting means of narrower width than said tape, while said tape is under substantially no longitudinal stress, said tape being retained in said restricting means until said tape has attained the desired cross-sectional configuration, said heated restricting means being of different shape than the ultimate configuration of the produced filamentary microtape, said tape being passed into said heated restricting means in such a manner that the longitudinal axis of said tape is centered in said restricting means, and (2) removing the shaped tape from said restricting means, followed by (3) cooling the so-formed filamentary microtape. That proposed method, however still does not seem to provide the rigid dimensional control that is needed to prepare filaments which will find use in textile operations.

Accordingly, it is the principal object of this invention to provide a process for preparing filamentary microtapes of labyrinthian cross section, which process provides substantially rigid and exact control of the microtape width and denier.

It is a further object of the invention to provide such a method utilizing a microporous tape whereby the tape is shaped and fused essentially simultaneously.

Another object is the provision of such a method utilizing a tape or ribbon of an organic, thermoplastic material.

The above and related objects are achieved in the method comprising (1) the passage of a fiat, flexible tape of an organic thermoplastic polymer into heated restricting means of narrower width than said tape, While said tape is under substantially no longitudinal stress, said tape being retained in said restricting means until said tape has attained the desired cross-sectional configuration, said heated restricting means being of different Shape than the ultimate configuration of the produced filamentary microtape, said tape being passed into said heated restricting means in such a manner that the longitudinal axis of said tape is centered in said restricting means, and (2) removing the shaped tape from said restricting means, followed by (3) cooling the so-formed filamentary microtape; by the improvement consisting of carrying out step (1) by passing said tapes into a circumferential groove in a roll, said groove having substantially parallel side walls, said walls being terraced to provide said groove with at least two widths.

The tapes useful in the present method may be of any organic thermoplastic resinous material. Materials which may be advantageously used are the normally crystalline polymeric materials. These are the polymers which have a tendency to form crystallites or sites where small segments of a plurality of the polymer chains are oriented and held in position by secondary valence forces. This crystallite formation or crystallinity is usually visible when the polymers are examined by X-ray diffraction. Typical of the normally crystalline polymeric materials falling within the advantageous definition are the polymers and copolymers of at least 70 percent by weight of vinylidene chloride with the remainder composed of one or more other monoethylenically unsaturated comonomers exemplary of which are vinyl chloride, vinyl acetate, vinyl propionate, acrylonitrile, alkyl and aralkyl acrylates having alkyl and aralkyl groups of up to about 8 carbon atoms, acrylic acid, acrylamide, vinyl alkyl ethers, vinyl alkyl ketones, acrolein, allyl esters and ethers, butadiene and chloroprene. Known ternary compositions also may be employed advantageously. Representative of such polymers are those composed of at least 70 percent by weight of vinylidene chloride with the remainder made up of, for example, acrolein and vinyl chloride, acrylic acid and acrylonitrile, alkyl acrylates and alkyl methacrylates, acrylonitrile and butadiene, acrylonitrile and itaconic acid, acrylonitrile and vinyl acetate, vinyl propionate or vinyl chloride, allyl esters or ethers and vinyl chloride, butadiene and vinyl acetate, vinyl propionate or vinyl chloride, and vinyl ethers and vinyl chloride. Quaternary polymers of similar monomer composition will also be known. It has been found that the normally crystalline copolymers composed of from about 92 to 99 percent by weight of vinylidene chloride and correspondingly from 8 to 1 percent by weight of acrylonitrile or of a lower alkyl acrylate have suitable polymerization characteristics, are well adapted for use in the manipulative steps in this process, and result in exceptionally useful filamentary articles. For these reasons, these vinylidene chlorideacrylonitrile and vinylidene chloride-lower alkyl acrylate copolymers represent preferred species for use herein. It should be understood, however, that the process is not limited to the treatment of tapes and ribbons of normally crystalline polymers but that those formed from any non-elastic, polymeric material may be used. There are many materials, such as polyvinyl chloride and polystyrene, which are capable of forming continuous, coherent articles, which are orientable, but do not normally form crystallites. The polymeric materials, whether crystalline or noncrystalline, may also include minor amounts of monomers, such as vinyl pyrrolidone, vinyl oxazolidinone, vinyl alkyl oxazolidinone, and the like, which are known to aid the dye-receptivity and other properties of fibrous materials. Likewise, polymers containing interpolymerized light and heat stabilizers may be used. Also operable in the present method are tapes and ribbons of polymeric materials, such as the polyolefins, including, for example, polyethylene, polypropylene, copolymers of ethylene and propylene, and polyisobutylene. Equally useful in the method are the tapes and ribbons formed from condensation polymers, such as the polyamides, including polyhexamethylene, diadiparnide, and the polyesters, including polyethyleneterephthalate. The limitation then on the material of which the tapes are made include thermoplasticity and an ability to be formed into a flat tape or ribbon.

The useful tapes for the present method are flexible tapes usually of about 0.001 to 0.005 inch in thickness and of about 0.1 to 1 inch in width. The thickness and width to be used in any given instance will depend in large measure upon the end product desired. The above limits are those which would normally be associated with the manufacture of filamentary microtapes finding their way into the textile art. When it is desired to make filamentary microtapes of greater size than, for example, about 0.3 inch, it would usually be found desirable to employ other known fabrication means, such as thermal extrusion, for their preparation. Wide sections of tape, which are more accurately referred to as films, are not handled conveniently in the present procedural steps. However, it should be understood that the process is not limited precisely to the 1-inch maximum, since useful articles may be prepared herefrom, although usually with less control of width and cross-sectional configuration than with the narrower tapes.

The tapes and ribbons finding use in the present invention are continuous but preferably unfused structures. By unfused is meant that the tape or ribbon is partially coalesced to have suflicient coherence to be self-supporting but requires thermal exposure above the temperature of plastic flow to attain that complete homogeneity of the fused article. Such continuous, unfused tapes and ribbons are preferred structures for use herein, because the method permits, and operates to greatest advantage with, simultaneous fusion and shaping. Such unfused tapes and ribbons frequently will have an inherent tendency to curl or to distort laterally under the influence of elevated temperature. The present process can make maximum use of that inherent tendency. Typical of such unfused structures are the coagula resulting from the continuous localized coagulation of a latex and the deposits resulting from the casting and drying of latex films. It should be apparent, however, that the method is also operable for preparing microtapes from those tapes and ribbons which are overtly fused to an extent which provides properties aceptable for the contemplated end use without further fusion.

As mentioned, the method of the present invention involves the passage in a particular manner of a fiat tape in planar fashion into confining and restricting means of a certain shape to cause the formation of the desired labyrinthian cross section. These restricting means areof smaller dimension than the width of the fiat tape passed therethrough. The confining and restricting means are of such a shape as to provide two different stresses to the tape, the restricting means will not have the shape or form of the ultimate microtape produced by the process. A particularly convenient and accordingly preferred means is a groove in the surface of a heated shaping device. With any given microtape the shaped and dimensions of the groove determine in large measure the shape and final dimensions of the microtape. For purposes of this invention, the groove walls are parallel and terraced so as to provide one groove but with at least two different widths. This is illustrated in FIGURE 1. The groove size desired to achieve a given end product will be able to be easily determined by simple preliminary experiments.

The manner in which the tape or ribbon is shaped is also critical. In the improvement of this invention, the forces applied to the tape are essentially completely laterally inward and are such that the edge portions of the tape will be rolled, folded, curled, or otherwise distorted from the original plane inwardly toward the longitudinal center of the ribbon. It should be apparent that a minimum amount of longitudinal tension must be applied to any flexible tape to maintain its planar integrity so that any lateral forces will be uniformly applied. However, for purposes of this invention, any longitudinal stress should be only such a minimum and not be of such magnitude as an orienting stress or similar force. This consideration is of most importance when the tape is a coagulum of a latex. Such a coagulum is weak and brittle and will break if any significant tensile stress is applied thereto.

The advantages and benefits, as well as the operation of the herein claimed method, will be more apparent from the following description and the appended drawings which illustrate a preferred procedural sequence for carrying out the method. In the drawings,

FIGURE 1 represents a typical groove configuration finding use in the invention and FIGURE 2 shows schematically a useful procedural sequence.

In the embodiment illustrated in FIGURE 2, a fiat tape is fed through a suitable guide 11, around a first pair of snubbing rolls 12, into a groove cut into the periphery of a rotating roll or drum 13. Roll 13 may be driven or may be an idler roll. In any case, roll 13 should rotate in the same direction as and with about the same speed as the tape 10. The snub rolls 12 and grooved roll '13 are so disposed that the flat tape 10 preferably enters the groove tangentially with the center line of the tape 10 in register with the center line of the groove. The tape is caused to travel about the roll 13 and is taken off tangentially by a second pair of snub rolls 14. After shaping, the microtape is passed about a third pair of snubbing rolls (not shown) operated at a greater peripheral speed than the second pair 14 of snubbing rolls to cause orientation therein. The so-formed microtape is then passed to a suitable wind-up means (not shown), throughother textile operations, or to any other desired point. The grooved roll 13 employed in the illustrated embodiment is a particularly convenient means for providing constant temperature, constant speed relative to the linear velocity of the tape, a relatively long length of travel in contact with the confining means, and is simple to construct and to change when a dilferent end product is desired.

As earlier mentioned, the shape and relative size of the groove compared to the flat tape determine to a great extent the configuration of the resulting microtape, even though the shape of the groove does not correspond to the final configuration of that microtape. In this respect, it should be noted that the groove configurations of this invention provide substantially better width and denier control than other configurations, such as those of prior disclosure.

It should be apparent that for the groove to accomplish its intended purpose there should be a force exerted downwardly toward the groove root on the tape to cause its descent into the root of the groove. With the preferred grooved roller, this downward force is inherent if the flat tape, while under constant minimum forwarding tension, is caused to enter tangentially and if the tape is caused to travel in an are so that the rise of the are between entering and take-01f is greater than the depth of the groove. However, it should be appreciated that this is the minimum length of travel. The actual are that is traveled by the tape will depend upon the amount of shaping desired, the heat setting necessary, the polymer being used, and similar factors. This may be determined by simple preliminary adjustments. The materials of which the confining means, such as the grooved roll, are constructed should be non-thermoplastic and non-heat distortable with a surface providing a ready release of the tape. For the organic, thermoplastic materials mentioned as finding utility in this invention, it has been found that a particularly desirable surface can be made of a steel grooved roll coated with solid polytetrafluoroethylene. Other materials will be suggested to the skilled worker. It should be understood that the invention is not limited to a grooved roll but that obvious mechanical equivalents, such as grooved continuous belts, may also be used.

As mentioned, the method of the present invention permits simultaneous fusion, shaping, and heat setting of the cross-sectional configuration. The heat used in the present method must be sufficient to fuse the tape. Thus, temperatures at which plastic flow of the polymer under the forces inherent in the process are required. These temperatures are characteristic of each polymer and will be known. The amount of heat needed in any given instance will depend in large measure on the material of which the tape is composed, the thickness and width of the tape, and the end product desired. Some polymers have higher softening temperatures than other polymers and, consequently, require a greater heat input for fusion or other thermally dependent treatment. Also, the amount of heat required to heat set a given configuration will vary with the cross-sectional geometry involved. When a grooved roll is used for a shaping device, it is most convenient to introduce heat transfer fluids through the trunnions of the roll itself. If desired, auxiliary heating means, such as radiant heaters mounted opposite the grooved roll may be employed.

The present method results in several benefits. It is capable of continuous operation. The method produces a higher tenacity tape than those obtained by other methods and the present method produces a tape of more uniform width than that obtainable by unrestrained curlings as by a thermal treatment without the restricting means or using grooves of different configurations.

It should be apparent that reproducibility of the folding is obtained only when the dimensions of the fused, fiat tape are essentially constant and the shaping conditions, such as the level at which the tape enters the groove, the depth to which it travels within the groove, and other factors, are maintained substantially constant. These constant conditions are easily attained by maintaining substantially constant minimum forwarding tension during the fusing and shaping steps. This minimum forwarding tension is attained with the second set of snubbing rolls and by having the restricting means travel at the same relative speed as the tape.

As mentioned, the process of the present invention permits the preparation of the microtapes from unfused, unoriented tapes or ribbons. This process results in several benefits. It it capable of continuous operation and is capable of being fitted into an integrated scheme that would include the tape or ribbon-making procedural sequence immediately preceding the present process. The microtapes resulting from the process of this invention are characterized by a higher tenacity and a better hand than the fiat tapes or ribbons formed from the same polymeric materials. The microtapes are further characterized by uniform width and denier and by the absence of any frayed, uneven, or torn edges.

By means of the present method and apparatus, it is possible to prepare microtapes of a wide variation in width/thickness ratio. Thus, microtapes having a width which is about two times greater than its thickness may be prepared with little deviation in this ratio. Such a filament will approximate the known monofilaments and fibers of oval cross section. However, with the same apparatus and with minor change in conditions, the width/ thickness ratio may be changed to 10 to 1 or greater. By merely changing the groove size, it is possible to attain width/thickness ratios of 30 to 1 or higher. Microtapes of such width/thickness ratios closely approximate fine ribbons or tapes and yet are subject to unusually exact control of width and denier and have edges which are non-frayable and have high tear strength.

The present process also is readily adaptable to the production of tapes of different deniers with minimum change in apparatus and conditions. When two or more flat tapes or ribbons are stacked and passed through the herein claimed procedural sequence, there results a microtape of similar configuration to that of a single tape but with proportionally increased thickness and denier. The widh of the so-lamniated or plied tapes is relatively modestly increased over that of the single, flat tape.

It will be appreciated that width/ thickness ratios, denier, and cross-sectioial configuration may be influenced by tape width and thickness, as well as the above-mentioned factors. The tape width can be adjusted with the slitting means. Tape thickness can be varied during its preparation or by the aforementioned plying techniques.

It can thus be seen that the process is susceptible in flexibility of operation to a diversity of microtape crosssectional configurations, deniers, and width/thickness ratios without the major retooling required by the prior known filament-making process when any change is contemplated.

The operation of the method will be more apparent from the following illustrative examples wherein all parts and percentages are by weight.

Example 1 A tape prepared by the continuous coagulation of a latex of a copolymer of 97 percent vinylidene chloride and 3 percent acrylonitrile and which was 0.0025 inch thick and 0.4 inch Wide was guided into a Aa-inch groove having parallel sides and out in a rotating steel drum of about 12.5 inches in diameter and coated with polytetrafiuoroethylene. The tape was so guided that the central longitudinal axis of the tape was centered in the groove. The drum was operated at a surface temperature of 180 C. The tape was fed onto the drum tangentially at a speed of approximately 50 feet per minute. The drum was rotated at a surface speed of about 50.5 feet per minute. A pair of water-cooled snub rolls removed the microtapes from the drum at a speed of 54 feet per minute. After cooling, the microtapes were passed about another set of snubbing rolls operated at a peripheral speed to impose a stretch ratio of '4 to 1 on the microtape. The fiat tapes, as prepared, were characterized in having a porosity gradient from the one surface to the other and in the present run the tapes were positioned so that the more dense side was toward the fusion drum. The tape was caused to travel around the drum for about 24 inches. The resulting microtapes had a configuration of the edge portions rolled over into a spiral section and the the center portion of the tape then folded over so that the curled edge portions were almost touching one another. The width of the microtapes was about 32.75 mils. The coeflicient of variation of the width was about 10.5 mils. When the process was repeated with everything identical except that the groove was terraced with the outward portion being .189 inch wide and the lower portion of the groove being .135 inch wide, the cofficient of variation was 2 /2 to 3 /2 mils.

What is claimed is:

1. In a method for preparing filamentary microtapes of labyrinthian cross-section comprising (1) the passage of a flat, flexible tape of a microporous coagulum resulting from the continuous localized coagulation of a latex of a normally crystalline polymeric material into restricting means heated to temperature of plastic flow of polymer and of narrower width than said coagulum while said coagulum is under substantially no longitudinal stress,

said tape being retained in said restircting means until said coagulum has been completely fused and the soformed tape has attained the desired cross-sectional configuration, said heated restiricting means being of different shape than the ultimate configuration of the proudced filamentary microtape, said tape being passed into said heated restricting means in such a manner that the longitudinal axis of said tape is centered in said restricting means, and (2) removing the shaped tape from said restricting means, followed by (3) cooling the so-formed filamentary microtape and (4) orienting said filamentary microtape by 1ongitudinal stress; the improvement consisting of carrying out step (1) by passing said tapes into a circumferential groove in a roll, said groove having substantially parallel side walls, said walls being terraced to provide said groove with at least two widths wherein a thermoplastic polymer is subjected to laterally applied stresses in increments of increasing magnitude to cause the edges of the coagulum to roll inwardly and provide a microtape having a. width equal to the width of the restricting means while at the same time applying sufficient heat to fuse the coagulant into a continuous film.

2. The method claimed in claim 1 wherein said normally crystalline polymeric material is a normally crystalline vinylidene chloride polymer.

3. The method claimed in claim 2 wherein said normally crystalline vinylidene chloride polymer is a copolymer of at least percent vinylidene chloride with the remainder of acrylonitrile.

4. The method claimed in claim 2 wherein said normally crystalline vinylidene chloride polymer is a copolyrner of at least 70 percent vinylidene chloride with the remainder of a lower alkyl acrylate having from 1 to 8 carbon atoms in the alkyl group.

5. The method claimed in claim 1 wherein said flat, flexible tape is from 0.001 to 0.005 inch in thickness and from 0.1 to 1 inch in width.

References Cited UNITED STATES PATENTS 3,194,7 16 7/ 1965 Lefevre 264--285 3,356,783 12/ 1967 Phillips 264-285 FOREIGN PATENTS 694,3 02 9/ 1964 Canada.

ROBERT F. WHITE, Primary Examiner.

J EFF ERY R. THURLOW, A ssistant Examiner.

US. Cl. X.R. 

